1. Field of the Invention
This invention relates to an electron gun which forms an electron beam using a cold cathode of the field emission cathode array (FEA) type and a microwave tube such as a traveling wave tube or a klystron for which the cold cathode electron gun is used.
2. Description of the Related Art
A conventional microwave tube will be described with reference to the drawings taking a traveling wave tube as an example.
As shown in FIG. 1, the conventional traveling wave tube includes a hot cathode electron gun 110 for forming an electron beam 108, an RF circuit unit 115 having a helix 116 for performing a mutual action between a microwave and the electron beam 108 to effect amplification of the microwave, and a collector 120 for catching the electron beam 108. The hot cathode electron gun 110 has a beam convergence ratio selected in most cases within the range from 15 to 40 in order to keep the current density of a cathode 101 at a suitable value to ensure long life, and an electron gun of the converging type called Pierce type is used for the hot cathode electron gun. In the hot cathode electron gun 110, the surface of the cathode 101 has a spherical shape, and a converging electric field is formed by a Wehnelt electrode 102 and an anode electrode 103 to converge the electron beam 108 while maintaining the laminar flow property of the electron beam 108.
For a cathode which serves as a source of generation of electrons, a cold cathode which does not require heating is available in addition to such a hot cathode as described above.
Next, a cold cathode of the field emission cathode array (FEA) type is described with reference to FIGS. 2a and 2b. FIG. 2a is a perspective view, partly cut away, of a conventional cold cathode of the FEA type, and FIG. 2b is a sectional view of essential part of the cold cathode of the FEA type. As seen from FIGS. 2a and 2b, an insulating layer 132 formed from a silicon oxide and a gate electrode 133 are layered on a silicon substrate 131. The insulating layer 132 and the gate electrode 133 are partly removed to form cavities 136 (see FIG. 2b), in each of which an emitter 134 having a pointed end is formed, thereby forming electron emitting sections 135 (see FIG. 2a). The electron emitting sections 135 are arranged in an array to form a cold cathode of the FEA type having a planar electron emitting area.
The thickness of the insulating layer 132 is approximately 1 .mu.m, and also the diameter of the openings of the gate electrode 133 is approximately 1 .mu.m. Further, the extremity of each emitter 134 has a radius of approximately 10 nm and is very sharp. Then, if the electric field strength becomes higher than 2 to 5.times.10.sup.7 V/cm, then electrons are emitted from the extremities of the emitters 134.
The cold cathode of the FEA type can realize a cold cathode current density which is five to ten times as high as that of a hot cathode. Further, since the cold cathode does not require heating different from the hot cathode, no heater power is required.
With a cold cathode electron gun of the FEA type, since the surface of the cathode is flat, where the beam convergence ratio is set equal to that of the hot cathode, if a converging electric field is formed by a Wehnelt electrode and an anode electrode similarly as in an electron gun of the Pierce type, then the trajectories of electron beams on the outer side of the cold cathode are curved more strongly to the center axis side than the trajectories of electron beams on the inner side of the cold cathode. Consequently, the beam trajectories cross over each other, resulting in loss of the laminar flow property of the electron beams. As a result, a stable electron beam having a low ripple percentage required for a microwave tube cannot be obtained. Besides, in the cold cathode of the FEA type, since each electron is emitted from the extremity of an emitter at an angle of approximately 10 to 30 degrees with respect to the direction of a normal to the surface of the cathode, electrons tend to expand in radial directions. As a result, convergence of the electron beams becomes more difficult.
In order to solve the problems described above, the applicant of the present application has proposed a cold cathode electron gun which employs an annular cold cathode in Japanese Patent Application No. 291453/96. FIG. 3 shows a sectional view including a center axis of the cold cathode electron gun proposed in Japanese Patent Application No. 291453/96 by the applicant of the present application.
Referring to FIG. 3, an annular cold cathode 141 has a large number of electron emitting sections 141a having a gate voltage(s) of E.sub.G, disposed annularly. Wehnelt electrodes 142 and 143 maintained at voltages Ew, and Ew.sub.2 are disposed at a central portion and a peripheral portion of the cold cathode 141. It is to be noted that reference numeral 144 denotes an anode electrode having a voltage of Ea. The ratio d/D between the inner diameter d and the outer diameter D of the region in which the electron emitting sections 141a are formed is higher than 0.8. By the construction just described, an influence of electron emitting angles from the extremities of emitters of the cold cathode 141 is moderated, and electron beams are converged from the inner side and the outer side by the two Wehnelt electrodes 142 and 143. Consequently, electron beam trajectories having a high laminar flow property and a low ripple percentage can be obtained. While the cold cathode 141 described above can be applied to an electron gun which has a comparatively high convergence ratio of approximately 20, the arrangement structure of the Wehnelt electrodes 142 and 143 is complicated, and this makes miniaturization and reduction in cost difficult.
Since the cold cathode of the FEA type can achieve a high current density comparing with the hot cathode, even if the convergence ratio is set lower than 10, usually there is no problem in regard to the life. Further, although the laminar flow property of electron beams is augmented by a decrease in convergence ratio, with an electron gun of the Pierce type, since the magnetic flux density in the proximity of the cathode increases in inverse proportion to the beam convergence ratio, if it is intended to decrease the convergence ratio, then the magnetic flux density in the proximity of the cathode must be increased. If the convergence ratio is set lower than 10, then the magnetic flux density usually becomes higher than several hundreds Gauss. Since such a high magnetic flux density cannot be obtained with the electron gun side leakage magnetic field of an electron beam converging magnet, an electron gun structure wherein the magnet is disposed outwardly of the electron gun cannot be avoided, and this makes the electron gun very large. Accordingly, an optimum structure of a cold cathode electron gun of the FEA type having a low convergence ratio is not known as yet.
Meanwhile, as an electron gun of a low convergence ratio, an example of a hot cathode electron gun for a low noise traveling wave tube is described below.
An electron gun having such a construction as shown in FIG. 4 is disclosed in A. S. Gilmour, Jr., "PRINCIPLES OF TRAVELING WAVE TUBES", Artech House, p.432. The electron gun shown in FIG. 4 includes an annular planar hot cathode 151, a Wehnelt electrode 152, and four anode electrodes 153, 154, 155 and 156. When the potential of the planar hot cathode 151 is 0 V, the potential of the Wehnelt electrode 152 is +12.5 V and the potential of the anode electrode 153 which is positioned nearest to the planar hot cathode 151 is +2.5 V. Further, in order to minimize standing waves of space-charge waves which propagate noise, where the potentials of the anode electrodes 153, 154, 155 and 156 are represented by Va1, Va2, Va3 and Va4 and the potential of a helix electrode (such as the one shown in FIG. 5) disposed forward of an end of the anode electrode 156 is represented by Ehe1, they have a relationship of Va1&lt;Va2&lt;Va3&lt;Va4&lt;Ehe1.
Further, on page 441 of the document mentioned above the characteristics of, such an electron gun as shown in FIG. 5 are disclosed. The electron gun includes a Wehnelt electrode and three anode electrodes. Reference symbols a, b, c, d and h in FIG. 5 denote the Wehnelt electrode, the first, second and third anode electrodes and the helix, respectively, and where the potentials of the anode electrodes and the helix are represented by Vb, Vc, Vd and Vh, respectively, they have a relationship of Vb&lt;Vc&lt;Vd&lt;Vh.
In the electron guns shown in FIGS. 4 and 5, in order to achieve noise reduction, a lateral direction velocity component of an electron beam must be eliminated. Therefore, the beam convergence ratio is set to approximately 1 and cannot be set positively higher than 1. Further, in order to minimize standing waves of space-charge waves which propagate noise, the potential distribution from the cathode to the helix must be made smooth, and usually, three or more anode electrodes are used.
Further, as an example of an electron gun which employs a planar cathode and a plurality of anode electrodes, an electron gun for a cathode ray tube is known.
For example, such an electron gun for a cathode ray tube as shown in FIG. 6 is disclosed in Japanese Patent Laid-Open Application No. 185659/82. The electron gun produces electron beam 168 using a beam forming electrode 166 and a converging electrode 165. The beam forming electrode 166 includes a planar hot electrode 161, a control grid electrode 162, a first screen grid electrode 163 and a second screen grid electrode 164. Where the potentials of the control grid electrode 162, first screen grid electrode 163, second screen grid electrode 164 and converging electrode 165 are represented by V1, V2, V2' and V3, respectively, they have relationships of V1=V2' and V2&gt;V2'. As concrete values, V1=V2'=0 V, V2=628 V, V3=690 V, and the cathode potential =47.5 V are disclosed. By setting the potentials of the control grid electrode 162 and the second screen grid electrode 164 lower than the cathode potential and besides setting the potential of the first screen grid electrode 163 to a value lower than 10 percent of the potential of the converging electrode 165 in this manner, there is an effect that expansion of electron beams 168 which have crossed over each other can be suppressed to make the beam spot diameter on the surface of the cathode ray tube small.
Meanwhile, such an electron gun for a cathode ray tube as shown in FIG. 7 is disclosed in Japanese Patent Laid-Open Application No. 163952/82. The electron gun includes a electron beam forming unit 174 including a planar hot cathode 171, a first grid electrode 172 and a second grid electrode 173, and a main converging lens unit 175 including a plurality of grid electrodes. The potential of the first grid electrode 172 is positive with respect to the cathode potential, and the potential of the second grid electrode 173 is negative with respect to the cathode potential. Due to the construction described, the electron gun has an effect that the degree of the cross-over of electron beams 178 can be reduced to reduce the beam spot diameter on the surface of the cathode ray tube.
As described above, an electron gun for a cathode ray tube includes a beam forming unit for causing electron beams to cross over each other, and a main converging unit for accelerating and converging the crossed over electron beams, and has totaling four or more electrodes.
In order to allow an electron gun for a microwave tube including a cold cathode of the FEA type to produce good electron beams having a low ripple percentage and having a suitable beam diameter, the following problems are involved.
First, where the beam convergence ratio is high, while an electron gun of a structure wherein Wehnelt electrodes are disposed on both of the inner and outer sides of a cold cathode having annular electron emitting sections is available, this structure is complicated and it is difficult to miniaturize and reduce the cost of the electron gun.
Second, where the beam convergence ratio is low, while an electron gun for a low noise traveling wave tube is suggestive, since the cathode current of the electron gun is several hundreds .mu.A and extremely low so as to achieve low noise operation, if the electron gun is applied as it is to an electron gun for a microwave tube which usually requires electric current of several tens mA or more, then electron beams are diverged by the space-charge forces of electrons and good electron beams cannot be obtained.
On the other hand, while also the electron gun structure for a cathode ray tube is suggestive as a planar cathode electron gun, since an electron gun for a cathode ray tube requires totaling four or more grid electrodes, the structure of the electron gun is complicated, and where it is applied to an electron gun for a microwave tube, it is difficult to achieve miniaturization and reduction in cost.
As described above, for an electron gun for a microwave tube which includes a cold cathode of the FEA type, a simple and small-size structure for obtaining good electron beam trajectories has not been established as yet.